Method and apparatus for concentrating vapors for analysis

ABSTRACT

A pre-concentration device and a method are disclosed for concentrating gaseous vapors for analysis. Vapors sorbed and concentrated within the bed of the pre-concentration device are thermally desorbed, achieving at least partial separation of the vapor mixtures. The pre-concentration device is suitable, e.g., for pre-concentration and sample injection, and provides greater resolution of peaks for vapors within vapor mixtures, yielding detection levels that are 10-10,000 times better than direct sampling and analysis systems. Features are particularly useful for continuous unattended monitoring applications. The invention finds application in conjunction with, e.g., analytical instruments where low detection limits for gaseous vapors are desirable.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of application Ser. No. 11,350,716,filed Feb. 8, 2006, now U.S. publication 20070180933A1, published Aug.9, 2007, now allowed.

This invention was made with Government support under ContractDE-AC05-76RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus forconcentrating gaseous vapors for analysis. The invention findsapplication in conjunction with, e.g., analytical instruments where lowdetection limits for gaseous vapors are desirable or wherepre-separation of vapors is desirable.

SUMMARY OF THE INVENTION

In one aspect, the invention is an apparatus for concentrating vaporsfor analysis, comprising: a conduit having at least one inlet and oneoutlet for flowing one or more vapors in a gaseous volume therethrough,said conduit containing a sorbent for concentrating vapors from saidgaseous volume by sorption, wherein said sorbent is operatively disposedwithin a metal foam. Typically a heater, or a heater and a temperaturesensor, is operatively disposed to heat the conduit, metal foam and thesorbent, which enables thermal desorption of vapors from the sorbent.

The metal foam provides a high thermal conductivity medium in which thesorbent is operatively disposed, so that when heat is applied to desorbvapors from the sorbent, the heat can travel rapidly from the peripheryof the conduit to the center. In this way, the entire sorbent bed can beheated more rapidly than would be observed if the metal foam were notpresent. For example, many sorbent materials are poor conductors ofheat, so that the center of a sorbent bed heats up more slowly than theperiphery when heat is applied to the periphery. Non-uniform heating candegrade the desired performance of a pre-concentrator. In one method ofuse, a vapor pre-concentrator, after collecting vapors on the sorbent,is heated as rapidly as possible to desorb the collected vapors as asingle brief pulse resulting in a tall, sharp peak in the detectedconcentration of the vapor. In this method, it is desirable that theentire bed of sorbent is heated as rapidly as possible. The presence ofthe metal foam can increase the rate of heating of the center of thesorbent bed thereby providing more rapid heating of the entire bed, anda sharper taller peak in the resulting vapor concentration. If thisvapor is delivered directly to a detector, the peak is taller, providingbetter sensitivity and lower detection limits. If the vapor is deliveredto a gas chromatograph, the shorter pulse of vapor at higherconcentration provides a shorter injection time and hence reduces bandbroadening due to injection.

In another method of using a pre-concentrator, the heat is delivered tothe sorbent more gradually, raising the temperature at a slowercontrolled rate. This provides a controlled thermal desorption of thevapors, in which some vapors may be desorbed at lower temperatures thanothers, resulting in a separation or partial separation of vapormixtures. If heat transfer within the sorbent bed is poor, the peripherywill heat faster than the center of the bed, and hence some regions ofthe bed will be at different temperatures than other regions of the bed.This will result in a longer period of time during which a vapor isdesorbed, yielding a broader desorption peak. When a mixture of vaporsis desorbed under these conditions, the broader peaks will lead topoorer separation and resolution of the individual vapors. By includinga metal foam in the design, the thermal desorption process occurs withbetter temperature uniformity throughout the sorbent bed. When theentire bed is at or near the same temperature at the same time duringthe thermal desorption process, the desorption peaks will be narrowerand individual vapors better separated and resolved.

So, whether one is heating the pre-concentrator as rapidly as possible,or heating at a slower controlled rate for a gradual thermal desorption,the inclusion of the metal foam in the pre-concentrator to provide morerapid and uniform heating of the entire bed volume is advantageous.Because metal foam is porous, the sorbent can be disposed within thepores and gases can flow therethrough. In fact, metal foam can be 95% ormore empty space by volume, providing an excellent flow through mediumwith excellent thermal conductivity, in which sorbent material can beplaced and gases can flow through.

In another aspect, the invention is a process for pre-concentratingvapors in a gaseous volume for analysis, comprising: providing a conduithaving at least one inlet and one outlet for flowing one or more vaporsin a gaseous volume therethrough, said conduit containing a sorbent forconcentrating vapors from said gaseous volume by sorption, wherein saidsorbent is operatively disposed within a metal foam; sorbing the one ormore vapors on the sorbent thereby concentrating the one or more vapors;and heating the sorbent to a temperature sufficient to desorb the one ormore concentrated vapors therefrom, generating a peak response in adetector of sufficient sensitivity for detection and/or analysis of theone or more concentrated vapors.

In another aspect, the rate of thermal desorption is controlled toprovide at least partial separation of the one or more vapors generatinga peak response in a detector of sufficient sensitivity for detectionand/or analysis of the one or more concentrated vapors.

In an embodiment, the conduit is a flow-through channel enclosed withina tube.

In another embodiment, a flow-through channel is a machined channel.

In another embodiment, the metal foam comprises nickel.

In another embodiment, the metal foam comprises stainless steel.

In another embodiment, the metal foam has a mean pore diameter of about380 microns.

In another embodiment, the metal foam comprises at least about 95% airby volume.

In another embodiment, the sorbent is of a mesh size selected in therange from about 60 to about 80.

In another embodiment, the sorbent is of a grain size selected in therange from about 170 microns to about 250 microns.

In another embodiment, the sorbent is selected from the group of Tenax®,Carbotrap, Carboxen, Carbosieve, glass bead), polymers, molecularsieves, activated carbons, carbon nanotubes, ceramics, Aluminas,Silicas, silica gels, polars, desiccants, or combinations thereof, orthe like.

In another embodiment, the apparatus is operatively disposed to adetector for measuring said one or more vapors desorbed therefrom.

In another embodiment, the apparatus is operatively disposed to adetector selected from the group consisting of Surface Acoustic Wavesensor, Flexural Plate Wave Sensor, mass spectrometer, ion mobilityspectrometer, gas chromatograph, sensor array, multivariate detector,flame ionization detector, chemical sensor, or the like, or combinationsthereof.

In another embodiment, the apparatus comprises a heater and atemperature sensor operatively disposed for controlled thermaldesorption of said one or more vapors concentrated therein.

In another embodiment, thermal desorption of said one or more vapors iseffected in conjunction with temperature programming and/or thermalramping.

In another embodiment, the apparatus further comprises a detector foranalysis of the one or more vapors desorbed therefrom selected from thegroup consisting of sensor arrays, chemical sensors, mass spectrometers,ion mobility spectrometers, gas-chromatography detectors, or the like,or combinations thereof.

In another embodiment, the apparatus includes a gas chromatographydetector selected from the group consisting of thermal conductivitydetectors, electron capture detectors, flame ionization detectors, orthe like, or combinations thereof.

In another embodiment, sorption and desorption of said one or morevapors occurs without altering composition of said vapors.

In another embodiment, the pre-concentrator provides an improvement indetection limit by a factor in the range from about 10 times to about10,000 times over that for a direct sampling and analysis system.

In another embodiment, the metal foam provides sufficient thermalconductivity to said conduit upon heating achieving essentially uniformtemperatures for rapid thermal desorption of said one or more vaporsconcentrated therein; and whereby desorption of said one or moreconcentrated vapors from said sorbent within said foam providessufficiently high sensitivity for detection of said one or moreconcentrated vapors.

In another embodiment, the sorption of said one or more vapors comprisesflowing said gaseous volume through said conduit at a rate in the rangefrom about 1 mL/min to about 10,000 mL/min for a period of time.

In another embodiment, heating of said sorbent is effected inconjunction with use of a heater and a temperature sensor operativelydisposed within said conduit for effecting controlled thermal desorptionof said one or more vapors.

In another embodiment, the signal modulation from said sensor overcomesdifficulties with baseline drift and re-zeroing of said sensor therebyfacilitating determination of the magnitude of the response from thestream of temporal data.

In another embodiment, detection of said one or more vapors in saiddetector provides for sample concentration and sample injection of saidone or more vapors, including a modulation of signal data resultingtherefrom.

In another embodiment, modulation of signal data provides forcontinuous, unattended monitoring applications.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following description of the accompanying drawing inwhich like numerals in different figures represent the same structuresor elements.

FIG. 1 is a cross-sectional view of a pre-concentrator apparatus,according to one embodiment of the invention.

FIG. 2 is a photomicrograph that illustrates the porous structure of ametal foam, where the pore size averages 380 microns across, accordingto an embodiment of the invention.

FIG. 3 illustrates a Wheatstone bridge used for feedback control ofthermal ramp rate and temperature measurement, according to anembodiment of the invention.

FIG. 4 illustrates a system for generating vapors and detecting vaporsin conjunction with a pre-concentrator, according to another embodimentof the invention.

FIGS. 5 a-5 b present thermal profiles of a pre-concentrator without andwith a metal foam core.

FIGS. 6 a-6 b present vapor mixture desorption results at varyingthermal ramp rates for a pre-concentrator with and without metal foam.

FIG. 7 compares mixture desorption profiles at 120 sec ramp time for apre-concentrator with and without metal foam.

FIG. 8 presents response profiles for a three-vapor mixture as afunction of pre-concentrator heating ramp rate.

FIG. 9 shows peak resolution data as a function of temperature ramptime, according to one embodiment of the process of the invention.

FIG. 10 presents response profiles for a tertiary vapor mixture ofMEK-TOL-DMMP at constant MEK-TOL and varying DMMP.

FIG. 11 is a plot of modeled peak data, using an exponentially-modifiedGaussian fit, according to one embodiment of the process of theinvention.

FIG. 12 compares modeled peak data from a ternary mixture compared withpeak data from a single vapor.

FIG. 13 compares modeled peak data using exponentially modified Gaussianmodel for curve fitting at varying thermal ramp times of 30, 60, and 120seconds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to a method and apparatus forconcentrating vapors for analysis. The term “vapor” as used hereinrefers to a substance in a gaseous state. Vapors include, but are notlimited to, e.g., inorganic vapors and organic vapors. In oneembodiment, the apparatus is a pre-concentrator wherein the addition ofthe pre-concentrator prior to real-time chemical sensor measurement ofvapors provides a means to automatically sample ambient gas therebyimproving measurement sensitivity. The term “Sensitivity” as used hereinis a measure of the amount of signal obtained for a given vaporconcentration, and is closely related to detection limit, the lowestconcentration that can be detected with a signal that is distinguishablefrom the noise. The term “Selectivity” as used herein refers to theability to distinguish one vapor from another in the one or more vaporsor analytes to be measured.

The apparatus of this invention comprises a conduit having at least oneinlet and one outlet for flowing one or more vapors in a gaseous volumetherethrough, said conduit containing a sorbent for concentrating vaporsfrom said gaseous volume by sorption, wherein said sorbent isoperatively disposed within a metal foam. In one aspect of theapparatus, the conduit is a tube containing a core of metal foam withsolid sorbent dispersed within the metal foam. A means is provided toflow the gas being sampled through the tube, collecting vapors on thesorbent by sorption. A means is provided to heat the tube to desorb thevapors, which under a flowing condition, exit the pre-concentrator wherethey are detected or analyzed by a detector or instrument. The inlet andoutlet can be interchangeable, for example, if the flow duringdesorption is in the opposite direction as the flow during collection,as is recognized in the art. Metal foam improves heating of the sorbentduring the vapor desorption process, by providing high thermalconductivity throughout the entire sorbent bed.

When large gaseous volumes are to be processed to capture their vapors,a sufficiently high cross-sectional area of the pre-concentrator isdesirable to facilitate the flow. In this case, there is a significantdistance from the periphery where the pre-concentrator tube is heated tothe center of the core of the pre-concentrator. Using metal foam as partof the core improves the thermal conductivity across this distance fromthe periphery to the core in a configuration that enables large flowrates to be used while collecting vapors from the sample.

Rapid thermal conduction throughout the sorbent (bed) also minimizes thepotential for localized overheating of sorbent that might result indecomposition and degradation of the sorbent.

In one aspect of the method, a small volume of solid sorbent collectsvapors from a large gas sample (e.g., at a given flow rate for a fixedperiod of time) and then releases the vapors into a small gas volumeduring thermal desorption. This process results in a concentratedchemical pulse that generates a peak in the detector response. Theinitiation of heating defines when analytes are delivered to ameasurement system. Signal modulation further overcomes difficultieswith baseline drift and sensor re-zeroing and facilitates automatedfeature extraction, i.e., determining the magnitude of the response fromthe temporal data stream.

Heating can be very fast to desorb vapors as rapidly as possible,providing a concentrated chemical pulse with the shortest duration andhighest chemical concentration. Alternatively, the heating may be lessrapid, at a controlled rate of heating, to provide programmed thermaldesorption of vapors from the sorbent, such that individual vapors arereleased at characteristic temperatures (or equivalently, characteristictimes during a thermal ramp) and mixtures of vapors are separated orpartially separated.

The method and apparatus find application in various functionsincluding, but not limited to, e.g., sampling, pre-concentration, sampleinjection, pre-separation, and signal modulation. Such features areparticularly useful for continuous monitoring applications with portableor unattended devices.

The pre-concentrator of the invention will now be described further inreference to FIG. 1.

FIG. 1 illustrates a cross-sectional view of a pre-concentrator 100 of abenchscale design for concentrating vapors for analysis, according toone embodiment of the invention. Pre-concentrator 100 comprises aconduit 10 enclosed and configured for flowing gases therethrough, e.g.,via an inlet 12 and an outlet 14. Conduit 10 can be comprised of variousmaterials, including, but not limited to, e.g., polymers, metals, glass,ceramics, or combinations thereof. No limitations are intended. In oneembodiment, conduit 10 is composed of a polymer, e.g.,polytetrafluoroethylene, also known as Teflon® (DuPont, Wilmington,Del., USA), with machined (e.g., Teflon) inserts 16. Conduit 10 includesa core 18 (bed) comprised of metal foam 20 with a sorbent 22 dispersedtherein. Wire mesh screens (not shown) at each end of the metal foamkeep the sorbent 22 within the foam 20. Inserts 16 are positioned tohold the wire mesh screens, metal foam 20, and sorbent 22 togetherwithin core 18. In addition, inserts 16 provide a means to connect inlet12 and outlet 14 to additional devices or systems, for example to adetector. Metal foam 20 of core 18 is porous (˜95% air), having a meanpore diameter of, e.g., 380 microns, but is not limited. In oneembodiment, core 18 comprises a cylinder of nickel foam 20 packed withsolid sorbent 22. FIG. 2 illustrates the pore structure of metal foam20.

Metal foam 20 of core 18 facilitates thermal conductivity andtemperature uniformity throughout core 18 of pre-concentrator 100 duringheating. The architecture of pre-concentrator 100 provides rapidtransfer of heat from the outer periphery of conduit 10 to the center ofcore 18. Metal foam 20 of core 18 further prevents agglomeration ofsorbent 22 particles into a single solid mass; metal foam 20 keepssorbent 22 particles dispersed therein.

In one embodiment, core 18 has dimensions approximately 12.7 mm long by4.8 mm diameter, consisting of two metal foam cylinders 4.8 mm indiameter cut from a sheet supplied at 6.35 mm thick, but is not limitedthereto. Core 18 comprises metal foam 20, solid sorbent 22 dispersedwithin metal foam 20, two wire mesh screens (e.g., Inconel 600 wire meshdisks with 120 mesh size, 0.094 mm from Tri Screen Inc, Claremont,Calif., USA), and two opposing Teflon® inserts 16 inserted into conduit20. In the instant embodiment, conduit 20 is a 58 mm length of Teflon®8-gauge shrink tube (e.g., Voltrex™ tubing, SPC Technology, Chicago,Ill., USA). Teflon® tubing is thermally shrunk to seal connectionsbetween the tube 20 and inserts 16. An additional layer of shrink tubingis used to secure a heater 24 (e.g., a model HK5573R5.1 Kapton foilheater, MINCO, Minneapolis, Minn., USA) and a thermocouple (e.g., amodel #5TC-TT-K-36-36-SMP-M, Type-K, thermocouple, OMEGA Engineering,Inc., Stamford, Conn., USA) around the outer diameter of conduit 10.With its Teflon® inserts 16 and Teflon® conduit 10, metal foam 20 ofcore 18 containing sorbent 22 is the most thermally conductive componentof pre-concentrator 100.

Sorbents

Choice of sorbent is not limited and depends in part on vapors to beconcentrated and the desorption temperatures desired, as will beunderstood and selected by those of skill in the art. No limitations areintended. Sorbents include, but are not limited to, e.g., resins (e.g.,Tenax-TA™, Carbotrap, Carboxen, Carbosieve, glass beads), polymers,molecular sieves, activated carbons, carbon nanotubes, ceramics,aluminas, silicas, silica gels, desiccants, or the like, or combinationsthereof. Sorbent particles may be of any size provided that flow ofvapors through pre-concentrator 100 is not restricted and particles canbe dispersed in the metal foam 20 of core 18. Mesh size is selected inthe range from about 30 (200 microns) to about 400 (37 microns), but isnot limited thereto.

In one exemplary embodiment, sorbent is composed of2,6-diphenylene-oxide, a porous polymer also known as Tenax-TA™available commercially (Scientific Instrument Services, Ringoes, N.J.,USA). Tenax-TA™ is preferably of a 60/80 mesh size, with a grain sizediameter of from about 170 microns to about 250 microns. In exemplarytests described further herein, core 18 was loaded with 25 mg Tenax-TA™but is not limited thereto. For example, the approach is extensible toother sorbents. For example, core 18 has been packed with Carboxen-569™sorbent (Supelco, Inc., Bellefonte, Pa., USA) in the metal foam 20,crushed and sieved to a 60/80 mesh size from an as-received 20/40 mesh.Thus, no limitations are intended.

Operation of the Pre-Concentrator

During operation, a small volume of sorbent collects vapors from a largegas sample (e.g., at a given flow rate for a fixed period of time) andthen releases the vapors into a small gas volume during thermaldesorption. Desorption provides a concentrated chemical pulse thatgenerates a response (signal) in a detector or instrument coupledthereto. The heating may be rapid, generating a brief concentratedchemical pulse containing the adsorbed vapors, or it may be less rapid,wherein the vapors are desorbed in a programmed thermal desorptionleading to separated or partially separated vapors. In either approach,using a highly porous metal foam to contain the sorbent and improvethermal conductivity throughout the sorbent bed is advantageous.

Controlled Thermal Desorption Via Use of Heaters

Because control of the thermal desorption of vapors is desired fromsorbent 22 present in core 18, an effective means of heating the entirecore 18 of conduit 10 is desired. Non-uniform heating and/or temperatureof sorbent 22 can result in nonuniform release of vapors during thethermal desorption process, leading to broader peaks. Pre-concentrator100 is thus preferably coupled to a heater 24. Heaters include, but arenot limited to, e.g., resistive heaters, band heaters, strip heaters,radiant heaters, multi-cell heaters, multi-coil heaters, polymerheaters, rapid response heaters, heat-exchangers, tape heaters, cableheaters, tubular heaters, cast-in heaters, cartridge heaters, ceramicfiber heaters, or the like. No limitations are intended. In oneembodiment (described in reference to FIG. 1), heater 24 is a resistiveheater 24 (e.g., a model HK5573R5.1 Kapton foil heater, MINCO,Minneapolis, Minn., USA) positioned (e.g., wrapped) around conduit 10for desorping vapors from sorbent 22 of core 18 of pre-concentrator 100.Resistive heater 24, having RTD properties, may simultaneously functionas a temperature sensor for monitoring and providing feedback control ofthe temperature

Pre-concentrator 100 is further optionally configured with one or morethermocouples for measuring and monitoring temperature during thermalcalibration and desorption experiments. Heater 24 provides for thermaldesorption of vapors from pre-concentrator 100. Heater 24 and a means ofmonitoring the temperature and adjusting the power to heater 24 providea means for controlled programmed thermal desorption of vapors frompre-concentrator 100.

In one exemplary embodiment, a simplified Wheatstone bridge circuit,illustrated in FIG. 3, is used to monitor and control temperature ofheater 24. In the instant embodiment, a resistive heater (heater 24 inFIG. 1), having resistance temperature detector (RTD) properties, servesas one leg of the bridge, such that the bridge balance voltage isdirectly proportional to the RTD resistance and thus temperature. Thebridge is initially balanced so that the bridge balance voltage is zeroat room temperature. Due to its resistance temperature detector (RTD)properties, heater 24 provides rapid heater temperature feedback.

A custom LabVIEW™ software program and a multifunction data acquisitioncard (e.g., a model 6030E PCI data acquisition card, NationalInstruments, Austin Tex.) is used to control the thermal ramp. An inputvoltage, V_(in), is supplied from a digital-to-analog converter (DAC) toraise temperature of heater 24. Bridge balance voltage, V_(out), ismeasured using an analog-to-digital converter (ADC). Input voltage isslewed programmatically to obtain a desired ramp rate and end pointwhile monitoring the bridge balance voltage at, e.g., a 35 Hz updaterate. Temperature of pre-concentrator 100 is ramped at controllablerates under software control by monitoring temperature and adjustingvoltage to heater 24 at an update rate of, e.g., 35 Hz. Increasing theinput voltage increases current through heater 24, increasing thetemperature, and changing its resistance. The change in resistance leadsto a change in bridge balance voltage which is monitored and fed into atemperature control algorithm in order to determine a subsequent inputvoltage. Calibration is provided by independently monitoring temperaturewith a thermocouple, which calibration is used with the bridge balancevoltage in a control algorithm. With this technique, pre-concentrator100 can be ramped from, e.g., room temperature to 170° C. in a linearfashion in 30 seconds or longer. No limitations are intended.Pre-concentrator 100 can be designed and controlled for faster or slowerlinear thermal ramps, or to ramp the temperature in controlled nonlinearways.

Control schemes employing separate heater and temperature sensorelements can suffer from thermal offset and overshoot. To minimize theseproblems, temperature control must be sufficiently damped. An overlydamped control approach, however, cannot provide sufficiently fastresponse needed to precisely control large thermal ramps because, e.g.,ΔT can be a range of up to about 200° C. Use of heater 24 as atemperature sensor for pre-concentrator 100 provides fast temperatureresponse to accurately control and provide fast thermal ramping. Inaddition, rapid heater RTD feedback facilitates fast thermal rampingwith minimal thermal offset or overshoot.

Thermal Ramps

Thermal ramp designs are not limited, as will be understood by those ofskill in the art. In one exemplary approach, thermal ramp is started ata preset ramp rate, ranging from about 0.8° C./sec to about 15° C./sec,to a first maximum preset temperature. The maximum temperature isselected to be sufficient to desorb all the vapors while not being sohigh that thermally induced degradation of the sorbent medium occurs. Atthe top of the thermal ramp, temperature is held for a preselectedperiod to allow test vapors to fully desorb. At the end of the ramp holdperiod and the vapor desorption, pre-concentrator 100 is allowed tothermally cool back to ambient temperature in preparation for the nextcycle. No limitations are intended.

Linear Temperature Ramping for Temporal Separation

Effect of thermal ramp rate on vapor mixture resolution using apractical pre-concentrator 100 design operated at multiple thermal ramprates is described herein. Temporal separation of desorbing vapors atvarying linear rates of temperature increase has been examined using aselected metric for resolution to determine if the rate of thermaldesorption influences the resolution achieved in a practicalpre-concentrator configuration. Pre-concentrator 100 provides goodthermal uniformity throughout the packed bed of sorbent in core 18. Peakresolution in the analysis of mixtures can be improved by decreasing thethermal ramp rate compared to heating of pre-concentrator 100 as rapidlyas possible.

Peak Resolution

The term “Peak Resolution” as used herein refers to the amount of peakoverlap for pairs of peaks or the ability quantify resolution. Anempirical formula for determining the peak resolution factor, R_(s), ameasure of peak overlap, and thus resolution, is calculated as a ratiocontaining the difference in position of peak maxima in the numeratorand the sum of the peak widths in the denominator, by equation [1]:R _(s)=1.18(t _(r)1−t _(r)2)/(pwhh1+pwhh2)  [1]

where t_(r) values are peak retention times for any two peaks. The valuefor R_(s) depends on the measure of peak width being used. Here, peakwidth is expressed in terms of peak width at half height (pwhh), but isnot limited thereto. Values obtained provide an indication of peakresolution, where higher values indicate less overlap between two peaks.For Gaussian peaks of equal height in chromatography, a value greaterthan 0.5 is required to observe separate peak maxima; a value greaterthan 1.5 indicates a baseline resolution. As values become larger,further baseline increases between peaks are observed.

Sensors

Choice of sensors and/or detectors for quantifying and detecting vaporsdesorbed from pre-concentrator 100 is not limited. Selection of sensorsand detectors depends on desired sensitivity, selectivity, and/or vaporsto be measured, as will be understood by those of skill in the art.Sensors and detectors include, but are not limited to, e.g.,multivariate detectors, chemometric detectors, sensor arrays, ionmobility spectrometers, mass-spectrometers, gas chromatographys, gaschromatography detectors, chemical sensors, or the like.

In one exemplary configuration, pre-concentrator 100 is connected to aflow cell containing a single polymer-coated microsensor as a detector,e.g., a Flexural Plate Wave (FPW) Sensor 204, to measure desorption ofvapors from pre-concentrator 100. The experimental apparatus is shown inFIG. 4. A vapor blending system, VBS, 202, is used to prepare dilutedvapor samples and samples containing mixtures of vapors and deliver themto pre-concentrator 100. FPW sensor 204 is located downstream frompre-concentrator 100.

FPW sensors mounted on separate alumina headers and oscillator circuitsare available commercially (Berkeley Micro Instruments, Berkeley,Calif., USA). In the instant configuration, the FPW sensor has aresonant frequency of 8 MHz, with etch pits 6 mm long and 0.5 mm wide,but is not limited thereto. The alumina headers contain a window overwhich the dies are mounted, with the FPW etch pit facing away from theheader and the metallized side exposed through the bottom of the header,where electrical connections are made. Prior to coating, FPW sensor 204is rinsed with dichloromethane and air dried, then cleaned with aUV-ozone device (e.g., a Model 342 UV-ozone Cleaner, Jelight Inc.,Irvine, Calif., USA) or cleaning method. FPW sensor 204 is spray-coatedwith a polydimethylsiloxane (PDMS) polymer to a frequency shift of 100kHz. The film is examined after being applied by optical microscopyusing a microscope (e.g., a Nikon Optiphot-M, OPTOTEK, Silicon Valley,Calif., USA) in conjunction with Nomarski reflected light differentialinterference contrast. In the instant embodiment, (coated) FPW sensor204 sensor is mounted in an aluminum flow cell (not shown) with a gasinlet and gas outlet, with the entire flow cell plus associatedelectronics maintained at 25° C. The inlet and outlet channels of theflow cell are placed at each end of the sensor etch pit, and the etchpit channel is used for flow over the sensing film thereby minimizingdead volumes. The flow cell is connected by 1/16″ Teflon tubing directlyto outlet 16 of pre-concentrator 100. In one exemplary approach,frequency data from the FPW sensor oscillator are recorded every twoseconds using a High Performance Universal Counter (e.g., a model 53131Auniversal counter, Hewlett-Packard, Palo Alto, Calif., USA) with amedium stability timebase, and data are transferred to a computer (e.g.,a Macintosh computer, Apple Computers, Cupertino, Calif., USA) via aIEEE-488 bus. Data are collected with, e.g., Labview™ software (NationalInstruments, Austin, Tex., USA)

For some experiments, another frequency data collection method was used.Frequency of FPW sensor 204 is measured, e.g., using a counter (e.g., amodel 6602 PCI counter configured with an 8 channel, 32-bit counter PCIcard having an internal 80 MHz timebase, National Instruments, Austin,Tex., USA). In exemplary tests, two counters are used. A first counter(Counter 0) counts a user-specified number of FPW frequency cycles, anda second counter (Counter 1) times how long it takes to count aspecified number of cycles, using the internal timebase as a clock.Measured frequency of the FPW signal represents the user-specifiednumber of cycles counted divided by the elapsed time required to countthem. The elapsed time is measured as the number of cycles counted onthe second counter divided by 80,000,000. The frequency measurements aretransferred directly to the computer CPU (e.g., via a 133 MHz PCI bus).Labview™ software reads, displays, and logs frequency data.

Use of Pre-Concentrator for Partial Separations

The operation of pre-concentrator 100 for partial separation of vaporsis fundamentally different from use of pre-concentrator 100 forpre-concentration and injection of vapors. In the latter case,desorption from pre-concentrator 100 is preferably fast and sharp toobtain the tallest detection peaks possible and to minimize bandbroadening from injection, e.g., when coupled with a chromatographicinstrument or step. When pre-separations are desired however, vapors arepreferably desorbed in a way that spreads them out over time.Accordingly, the influence of thermal ramp rate on partial separation ofvapor mixtures is a factor in the configuration and method of useselected for pre-concentrator 100.

For example, in one exemplary embodiment of an advanced instrument forvapor detection, pre-concentrator 100 can be coupled to a multivariatedetector. In the instant embodiment, use of a multivariate detector suchas a sensor array in combination with pre-concentrator 100 providing atleast partial temporal vapor separation creates a system that can takeadvantage of advanced chemometric methods for better chemicalinformation extraction. Addition of pre-concentrator 100 to a sensorarray system, and processing the data with advanced chemometrics methodscan overcome some of the limitations of an array alone, even ifpre-separation achieved with pre-concentrator 100 does not providebaseline resolution (i.e., resolution values of 1.5 or better).Mathematically, such an approach represents a second-order system oneobtains two vectors of data per sample. By contrast, a sensor arrayalone is a first order measurement that provides only one vector of dataper sample. Additional information provided by a second-order systemconfiguration can provide improved selectivity and the potential toquantify vapors and/or analytes in the presence of unknowninterferences.

The use of a pre-concentrator incorporating metal foam is advantageousfor development of a second order system because it can provide a betterpre-separation than an equivalent pre-concentrator lacking the metalfoam. The better pre-separation in front of the multivariate detector,which results from better temperature uniformity of the bed duringcontrolled thermal desorption, provides better data for processing bysecond order chemometric methods.

In addition, use of up front separation in conjunction with amultivariate detector allows use of multivariate curve resolutiontechniques that can mathematically resolve overlapping peaks, even atlow resolutions. Such methods can be used to quantify analytes inmixtures and to obtain pure component spectra, even at resolutions below0.5 where separate peak maxima are not seen. Hence, there is great valuein obtaining even partial temporal resolution of vapor mixtures.

The ability to obtain pure component patterns from multivariate curveresolution techniques further enables use of new classificationtechniques. For example, pure component vapor patterns can be used in aclassification method that transforms such vapor patterns intodescriptors of the vapor solubility properties for sorbed vapors sorbedas a mass, as a volume, and for mixtures. Thus, using apre-separator/array instrument and multivariate curve resolution, purecomponent patterns can be extracted which can then be classified.

The following examples are intended to promote a further understandingof the present invention. Example 1 describes results for apre-concentrator with and without metal foam. Example 2 describes vapormixture desorption experiments in conjunction with pre-concentrator 100where vapors are desorbed at varying thermal ramp rates leading toimproved mixture resolution. Example 3 describes results for vaporseparation tests using pre-concentrator 100 for a tertiary mixture ofMEK-TOL-DMMP vapors, with varying DMMP concentrations

EXAMPLE 1 Pre-Concentrators With and Without Metal Foam Core

Example 1 describes thermal characteristics of a pre-concentratorcontaining metal foam 20 in core 18 of pre-concentrator 100 incomparison to one lacking metal foam, which demonstrates that the metalfoam improves thermal conductivity and thermal uniformity in the sorbentbed. In addition, it has been observed in vapor desorption tests thatpre-concentrators without metal foam lead to broader more overlappedpeaks from vapor mixtures. Thus the metal foam also provides narrowerbetter resolved vapor desorption peaks.

Experimental. Pre-concentrator 100 with core 18 of metal foam 20 waspacked with ˜25 mg of 60/80 mesh Tenax-TA™ sorbent (Supelco, St. Louis,Mo., USA) 22. Metal foam 20 used was a porous nickel foam (e.g.,Ampormat 200 series, Astro Met Inc., Cincinnati, Ohio, USA) having apore density of about 60 pores per inch in sheets. Pre-concentratorsprepared without metal foam 20 in the core were also prepared forcomparison purposes. The same quantity (˜25 mg) of sorbent (e.g.,Tenax®) 22 was packed into the pre-concentrator lacking metal foam aswas packed into the pre-concentrator that contained metal foam. Teflon®adapters in the pre-concentrators lacking metal foam were insertedfurther into conduit 10 as the volume of Tenax® sorbent 22 particlesalone was slightly lower than that observed for core 18 ofpre-concentrator 100 including metal foam 20. Thermocouples (e.g., model#5TC-TT-K-36-36-SMP-M, Type-K thermocouples, OMEGA Engineering, Inc.,Stamford, Conn., USA) were inserted into the pre-concentrators tomeasure temperatures at the center and periphery of the core 18 of each.Thermocouples had a head diameter of about 250 microns, comparing withTenax® particles of from about 170 to about 250 microns. In addition, athermocouple was present on the outside of pre-concentrator 100 conduit20, as usual. The resistive heater 24 of pre-concentrator 100 wasincluded as a leg of a simplified Wheatstone bridge, as describedhereinabove with reference to FIG. 3. Increasing the input voltageincreased current through heater 24, increasing the temperature, andthus changing its resistance. The change in resistance led to a changein bridge balance voltage which was monitored and fed into a temperaturecontrol algorithm to determine a subsequent input voltage.Pre-concentrator 100 temperatures were ramped at controllable ratesunder software control by monitoring temperature and adjusting voltageof heater 24 at an update rate of 35 Hz. FIGS. 5 a and 5 b comparethermal characteristics of core 18 of pre-concentrator 100 with andwithout metal foam 20. Results are shown for a 60 sec temperature ramp.

Results. As illustrated in FIGS. 5 a and 5 b, significant and observabletemperature differences are noted between the center and periphery ofthe cores of pre-concentrators without metal foam as pre-concentrator100 heats up. As illustrated in FIG. 5 a, in the absence of metal foam20, as the periphery of core 18 heats to the heater temperature, over 10degrees difference exists in temperature observed across the radius ofcore 18, e.g., between the periphery and center of core 18. Asillustrated in FIG. 5 b, when metal foam 20 is present in core 18,temperature at the center of core 18 closely tracks temperature observedat the periphery of core 18. Core 18 of pre-concentrator 100 with metalfoam thus maintains a more uniform temperature across its radius. Thus,the results show that the metal foam improves the heating throughout thesorbent bed of the pre-concentrator and provides more uniform sorbentbed temperatures.

Pre-concentrators with and without metal foam were also used to collectvapor mixtures and desorb them to a sensor. In the absence of metal foam20, pre-concentrators yielded broader, more overlapped peaks duringdesorption of vapor mixtures. Pre-concentrators 100 with metal foamprovided narrower peaks and visibly better peak resolution. Results areshown in FIGS. 6 a, 6 b, and 7. Pre-concentrators with and without metalfoam were prepared as described above, each with 25 mg of Tenax® sorbent22. Pre-concentrator 100 was placed in series between a vapor generator202 (e.g., VBS system) and a single FPW sensor 204 acting as detector ina system 200 described previously in reference to FIG. 4. Ternarymixtures of vapors were generated consisting of 2240 mg/m³ methyl ethylketone (MEK), 220 mg/m³ toluene (TOL), and 82 mg/m³ dimethylmethylphosphonate (DMMP), obtained from Aldrich (St. Louis, Mo., USA).Vapors used are representative of vapors having differentfunctionalities and volatility in order to demonstrate varyingdesorption rates from pre-concentrator 100. Flow rates during theexperiments were 25 mL/min and the pre-concentrators were heated to 200°C. during the desorption process. FIGS. 6 a and 6 b show desorptionprofiles for ternary mixtures at various thermal ramp rates. Bycomparing the desorption peaks observed in FIG. 6 a for thepre-concentrator containing metal foam with those in FIG. 6 b for thepre-concentrator lacking metal foam, it is seen the peaks are betterresolved using metal foam. FIG. 7 compares results for pre-concentratorswith and without metal foam in one figure, selecting the 120 second ramptime as an example. It is clearly seen that with metal foam the peaksare taller and more symmetrical and better resolved than seen withoutmetal foam.

EXAMPLE 2 Vapor Separation of a Tertiary Mixture as a Function ofHeating Ramp Rate

Example 2 describes test results using pre-concentrator 100 containingmetal foam for vapor separation of a tertiary mixture of MEK-TOL-DMMPvapors. Peak separation and resolution were demonstrated and measured byvarying the controlled thermal ramp rate of pre-concentrator 100 asdescribed herein.

Experimental. Pre-concentrator 100 was placed in series between a vaporgenerator 202 (e.g., VBS system) and a single FPW sensor 204 acting asdetector as described previously. Ternary mixtures of vapors weregenerated consisting of methyl ethyl ketone (MEK), toluene (TOL), anddimethyl methylphosphonate (DMMP), obtained from Aldrich (St. Louis,Mo., USA), representing vapors having different functionalities andvolatility in order to demonstrate varying desorption rates frompre-concentrator 100. Vapor concentrations of the tertiary mixture ofvapors were 2230 mg/m³ MEK, 220 mg/m³ TOL, and 110 mg/m³ DMMP,respectively. A two minute vapor collection period was employed. Vaporsdesorbed from pre-concentrator 100 were detected using a singlepolymer-coated FPW sensor 204 in a low dead volume flow cell. Fourthermal ramp rates were using ranging from about 15° C./sec (10 secramp) to about 0.83° C./sec (180 sec ramp), i.e., at 10 sec., 60 sec.,120 sec., and 180 sec., respectively. Maximum ramp temperature was ˜170°C. Flow rate of nitrogen sweep gas was 100 sccm. FIG. 8 shows the effectof thermal ramp rate on peak shape and separation provided bypre-concentrator 100 as a function of heating ramp rate. Responseprofiles are shown for the three-vapor mixture. Thermal profiles for thefour thermal ramp rates are shown in the lower traces (referenced to theright y-axis). The thermal ramp was programmed to be linear at all fourthermal ramp rates. Vapor peaks detected by FPW sensor 204 are shown inthe upper traces (referenced to the left y-axis) resulting in a familyof curve traces. Traces are offset for clarity. Resolution results areshown in FIG. 9. Peak resolutions were calculated according to a metricfor resolution (R_(s)) described hereinabove.

Results. In FIG. 8, actual thermal ramps are linear for the 60, 120, and180 second ramps. Vapors are released in the order of MEK, TOL, andDMMP. As thermal ramp rate becomes slower, positions of the peak maximashift to longer time positions and are more widely separated. At thesame time, peak heights become lower as the peaks become broader. InFIG. 9, peak resolutions increase as a function of temperature ramp timeup to a ramp time of 120 seconds. The left axis shows R_(s), the peakresolutions, plotted with reference to the left y-axis where squarescorrespond to resolution of MEK and TOL and circles correspond toresolution between TOL and DMMP. Peak heights referenced to the righty-axis are included for comparison. The peak heights as a function oframp time are shown as dashed lines referenced to the right y-axis,where the triangles pointing up are for DMMP, the triangles pointingdown are for TOL, and the diamonds are for MEK. Resolution of the vapormixture improves with increasing thermal ramp time up to about 120seconds. Resolution improves even as peaks become broader.Pre-concentrator 100 in combination with programmed thermal desorptioncan thus be used as a pre-separator in addition to its usual functionsfor sampling, signal modulation, and improving sensitivity. As specifiedin example 1, pre-concentrators without metal foam result in broaderpeaks and less resolution of vapor mixtures, and hence are of less useas a pre-separator.

EXAMPLE 3 Pre-Concentration of Varying Vapor Concentration in a TertiaryMixture

Example 3 describes results from vapor separation tests usingpre-concentrator 100 involving tertiary mixtures of MEK-TOL-DMMP vaporsat constant MEK-TOL with varying concentrations of DMMP. Tests on binarymixtures of MEK-TOL and TOL-DMMP were also conducted for comparisonpurposes.

Experimental. Ternary mixtures of vapors were generated consisting ofmethyl ethyl ketone (MEK), toluene (TOL), and dimethyl methylphosphonate(DMMP) as described in Example 2. Concentrations were held constant forMEK and TOL at 2230 mg/m³ and 220 mg/m³, respectively. DMMP vaporconcentrations were varied, ranging from 6.9, 13.7, 27.4, 54.8, and 110mg/m³. The MEK-TOL-DMMP vapor mixtures were pre-concentrated usingpre-concentrator 100 as described in Example 2. FIG. 10 plots theresponse profiles at a single thermal ramp rate of 2.5° C./sec (60 secramp). Ramp temperature is overlaid, referenced on the right handY-axis.

Results. In FIG. 10, the effect of varying one component (DMMP) whilemaintaining the other components (MEK and TOL) constant is demonstrated.MEK and TOL peak shapes and separation are unaffected by changing thethird component (DMMP); peak areas and peak heights for DMMP increasewith increasing DMMP test concentration as expected. Results describedpreviously in Example 2 for tests on tertiary mixtures of MEK-TOL-DMMPdid not show significant differences from those in binary mixture testsof MEK-TOL and TOL-DMMP. Peak separation times between the pairs ofpeaks in both the binary mixture tests and tertiary mixture tests arecomparable; no significant differences are observed between testresults. Addition of a third component did not change the respectivepeak parameters for the other two components. This example demonstratesthat when the pre-concentrator apparatus is designed and operated toprovide separations, it is possible to monitor varying concentrations ofa component even in the presence of other components in the mixture.

When vapors are desorbed as partially overlapping peaks, curve fittingtechniques can be used to extract individual peak profiles, and hencedetermine the peak heights and peak areas of the individual vapors. Whena univariate sensor is used, conventional curve fitting techniques knownin the art can be used. FIG. 11 shows peak fitting for data collectedusing pre-concentrator 100 containing metal foam in conjunction with FPWsensor 204 as described hereinabove. The data points are as opencircles. The solid curve shows the peak fitting, and the underlying peaktraces show the individually modeled peaks. An exponentially modifiedGaussian peak shape was used for modeling and peak fitting. FIG. 12shows a ternary mixture, the modeled DMMP peak from the mixture, and anexperimental trace from an experiment with DMMP that is not in a vapormixture. Peak areas for DMMP in mixtures were determined by this methodin calibration experiments at multiple DMMP concentrations and variousthermal ramp rates. FIG. 13 shows that similar peak areas are obtainedat each concentration regardless of the thermal ramp rate. Therefore,peak areas can be used to quantify the amount of DMMP in the sample evenwhen the sample is in a mixture, provided that one obtains sufficientpre-separation.

Such curve fitting approaches improve in precision and accuracy thebetter the peaks in the mixture are resolved. Since the metal foamimproves the peak resolution, it improves the precision and accuracywith which vapors can be quantified in mixtures when detected by aunivariate detector.

If the detector is a multivariate detector, then multivariate curveresolution methods can be used to extract pure component concentrationprofiles. These curve fitting approaches also improve in precision andaccuracy the better the peaks in the mixture are resolved. Since themetal foam improves the peak resolution, it improves the precision andaccuracy with which vapors can be quantified in mixtures when detectedby a multivariate detector

While the preferred embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its true scope and broader aspects. The appended claims aretherefore intended to cover all such changes and modifications as fallwithin the spirit and scope of the invention.

1. A method for performing analysis of gases present in a gaseousmixture, characterized by the steps of: collecting gases within a tubedefining a conduit having at least one inlet and one outlet flowing aplurality of vapors in a gaseous volume therethrough, said conduit isfilled with a metal foam core containing at least one sorbent disposedwithin said metal foam, said sorbent is configured to selectively adsorband desorb preselected materials at designated conditions, said metalfoam is further configured to have sufficient thermal conductivitywhereby essentially uniform temperatures are obtained throughout saidmetal foam in response to heating or cooling; sorbing said plurality ofvapors on said sorbent within said metal foam to concentrate saidplurality of vapors therein; and heating said sorbent to a temperaturesufficient to desorb said plurality of concentrated vapors therefrom,said desorption generating a response of sufficient sensitivity in adetector for detection and analysis of said plurality of concentratedvapors.
 2. The method of claim 1, wherein said sorbent is selected fromthe group consisting of Tenax®, Carbotrap, Carboxen, Carbosieve, carbonnanotubes, glass bead, polymers, molecular sieves, activated carbons,ceramics, Aluminas, Silicas, silica gels, polars, desiccants, andcombinations thereof.
 3. The method of claim 1, wherein the sorption ofsaid plurality of vapors comprises flowing said gaseous volume throughsaid conduit at a rate in the range from about 1 mL/min to about 10,000mL/min.
 4. The method of claim 1, wherein the step of heating saidsorbent is performed using a heater and a temperature sensor operativelydisposed within said conduit for effecting controlled thermal desorptionof said plurality of vapors that provides at least partial separation ofsaid plurality of vapors.
 5. The method of claim 1, wherein the thermaldesorption of said plurality of vapors is performed using temperatureprogramming and/or thermal ramping.
 6. A method for preconcentratinggases in a gaseous volume for analysis thereof, said methodcharacterized by the steps of: providing a concentrator having at leastone inlet and one outlet for flowing a gaseous volume containing aplurality of gases therethrough, said concentrator includes a centralcore filled with a metal foam containing at least one sorbent dispersedwithin said metal foam; passing said gaseous volume through said metalfoam to adsorb said gases on said at least one sorbent concentratingsaid plurality of gases therein; and desorbing said concentrated gasesuniformly from said at least one sorbent at a concentration sufficientto generate a response in a detector of sufficient sensitivity in apreselected time for determination of said plurality of gases.
 7. Themethod of claim 6, wherein said metal foam has a thermal conductivitysufficient to provide essentially uniform temperature throughout saidmetal foam core in response to heating or cooling.